Currently Creo Simulate/Mechanica will not allow you to create additional or control current assembly level cross sections while in Simulate/Mechanica. This causes you to have to jump in and out of Simulate/Mechanica mode in order to turn on/off existing assembly cross sections or to create additional cross sections.
The need arrives when dealing with large assemblies where there are buried parts/components in the assembly that are not easily visible. Having access to creating or turning on existing cross sections in the assembly allows you to easily apply contact, free, bond and other constraints between surfaces of buried components.
